DSS044: Preventing Storage Silo Explosions During Firefighting Efforts

In this episode of the DustSafetyScience Podcast, we’re talking about preventing storage silo explosions during firefighting efforts. It is a highly important topic, as we’re continuing to see dust fire response as an issue, especially in storage silos. Off gases can collect at the top of the silo and cause an explosion if ignited. An explosion can also occur if the dust is stirred up. These are only two of the potential outcomes that call for further discussion.

In this episode, we will be discussing:

The different types of silos
How silo fires get started
Why they are so difficult to fight
How they escalate
Steps in the firefighting response
Preventative measures to take

A lot of the material we cover in this episode is based on a report called Silo Fires: Fire Extinguishing and Preventive and Preparatory Measures. It was written by Henry Persson from the SP Technical Research Institute of Sweden. A lot of groups, including WorkSafeBC and the Wood Pellet Association of Canada, have used his research to guide their own response to storage silo fires.

Overview of Storage Silo Fires

We’ll start with a general overview of storage silo fires. We covered a lot of this information in Episode #8 with Todd Siefke of WorkSafe BC, but below is a summary for the purposes of this episode.

What Are The Different Kinds of Storage Silos?

Different types include:

Tower silos, which are high concrete structures with a comparatively small diameter. They are usually grouped close together in a silo cell network. They can present access challenges when you’re fighting a fire inside.

Freestanding silos, which have a larger diameter and are made of steel or concrete. Some industries refer to them as bins.

Box silos, which are smaller and often seen at agricultural sites, where they are used to temporarily store products that are about to be loaded onto trucks.

Oxygen-limiting silos, which are completely sealed off to avoid oxidation of the materials that are inside.

Silage silos, which are steel structures approximately five to eight meters in diameter. Filling and extracting are typically done from the top.

How Do Silo Fires Get Started?

These fires get started a number of ways. One is by smoldering combustion. If material sitting inside a silo has moisture levels that are too high, it will start to smolder. There are also instances where the material is already smoldering or on fire and more is dumped on it, leading to a deep-seated fire in the silo.

These fires tend to move around as they burn. They may move down into the silo. If you’re extracting or removing material, the way it will move depends on the bulk solid flow characteristics. If there is a core flow, the outcome will be different than if material was being taken out of one side. You may not be able to predict how that smoldering fire is going to react, which is a big point that’s highlighted in the report.

Why Are Silo Fires So Difficult To Fight?

These fires present difficulties for multiple reasons. One is that it can be hard to identify where the fire is. It’s also hard to pinpoint how it moves with time, and some fires may not be noticed until they’re well advanced.

How Do Silo Fires Escalate?

Silo fires can escalate under multiple circumstances. As the material is burning, it releases combustion gases. Due to an incomplete combustion, these materials may have fuel left in them, such as carbon monoxide, which can build up in the headspace. When firefighters open the hatch or cut their way through the roof, they can let in enough oxygen to cause a gas explosion.

Another risk is stirring up dust and causing a dust explosion. There are also cases where a dangerous hybrid mixture in the space leads to an explosion, so in general, any oxygen entrainment presents a hazard. If you’re fighting a silo fire and cut into the side of the structure, you can let in oxygen that can cause an explosion or, at the very least, enhance how long the fire burns.

According to Henry Persson’s report, if you open a hatch to put the fire out (while discharging the silo contents), a flashover explosion could potentially result, causing personal injury and destroying the silo and its contents. The recommended strategy is to close things off.

Another way that incidents escalate is when people enter the silo to fight the fire. The problem is that the combustive gases inside may not contain enough oxygen for them to breathe and large quantities of carbon monoxide could be present.

There are a couple of traditional firefighting methods that can cause silo fires to escalate:

Spraying water inside the silo. Material such as wood pellets can expand as it absorbs the water. In a worst-case scenario, it can put pressure on the outer walls and collapse the silo. Not only is this outcome dangerous for people in the vicinity, but it can also lead to a dust explosion.

Using carbon dioxide to inert the headspace. CO2 is often used to inert the headspace, which has a couple of dangers associated with it. One is that it can create more carbon monoxide gas, which is why nitrogen is recommended instead. Research also suggests that spraying carbon dioxide gas into the headspace of a burning silo can actually cause an explosion.

What Is The Recommended Approach For Fighting Silo Fires?

If your silo is on fire, what should you do? The Persson report recommends 13 steps that can help you identify and manage the hazard.

Identify the silo and fire types. What type of silo is it? Does it limit oxygen or have openings that may be allowing combustible gases to escape? With regards to the fire, when was it noticed? How long was it burning? Is there any way to determine its size?
Carry out an initial risk assessment. After identifying the silo type and the fire scenario, determine whether or not areas within the structure have reduced oxygen concentration or increased carbon monoxide concentration.
Consider the risk of a gas or dust explosion. You don’t want to position the response team anywhere that could be impacted by an explosion.
Close the silo to minimize any air entrainment. Seal openings and shut off ventilation systems. If oxygen gets into the silo, it can escalate the fire’s progress. You also don’t want inerting gases escaping from the structure during firefighting.
Requisition nitrogen gas and equipment. Nitrogen is the preferred gas for fighting silo fires. It is delivered as liquid, so you will need vaporization equipment.
Inject nitrogen from the bottom of the silo. Although silo fires have traditionally been fought from the top, it can be dangerous to have workers up there. It’s also not an efficient approach because you have to push combustible gases down to get the inerting gas in. If you inject from the bottom, it’s much easier to push those gases up to the top.
Assemble gas measuring equipment. This equipment enables you to determine the levels of oxygen and CO2 in the headspace and assess explosion risk.
Apply foam to the headspace if necessary. If you have an open surface fire in the headspace, it can burn the top of the silo and be carried into the adjacent silos or conveyor systems. Applying foam can help prevent this outcome, but the key is to minimize the amount of oxygen you allow in during the process. Any responders at the top of the silo also need to be wearing protective gear.
Inert the silo. Use lances to inert the silo and pump in nitrogen until you get a quenched condition in your silo, meaning that the environment is inerted and can’t sustain combustion.
Discharge the material once the fire is under control. Once gas measurements hit a certain level, discharge the material.
Go slowly. Plan for a long discharge operation. Go slowly to avoid stirring up dust and potentially causing an explosion.
Sort discharged material as it comes out. Sort the discharged silo contents into two piles- burned material and material that’s still good. Check both piles to confirm that nothing is smoldering.
Keep injecting the gas during discharge. As you pull the material out, maintain the gas flow so that the oxygen concentration in the silo does not exceed 5%. That way, you can prevent the combustible material from flaring up and causing an explosion.

Here are some additional cautionary tips:

Never enter the building without gas monitoring equipment or breathing apparatus, as carbon monoxide is present in elevated concentration levels and oxygen concentration is lower.
Water should not be used within the silo, especially if it contains material like pellets, because the silo can topple from the swelling material or pressure from the water.
Liquid nitrogen is freezing cold, so wear protective gear and use the appropriate apparatus to avoid a frostbite injury.

Preventative Measures To Take

The report concludes with some preventative and preparatory measures, some of which we review below.

- Install a fixed gas distribution system. If you put in a piping system when the silos are being built, you get much better flow rates and can quench the fire more quickly.
- Install a system to easily and safely inert the silo headspace. This measure can greatly reduce the number of people injured in silo fires because it prevents oxygen from getting in and there is no need for teams to go to the top of the silo to inspect the headspace.

Ensure access to mobile gas equipment and resources. Remote facilities may not have access to nitrogen or another inerting gas. Some industries are investing in this equipment and maintaining it in a central location, where it can be distributed to local facilities at a moment’s notice.

Conclusion

Preventing storage silo explosions during firefighting efforts sounds like a complicated process. That’s because it is. Responding to a fire in a silo takes a lot of time and skill. It also requires that the response team, the owners of the silo, and the industry operators all be on the same page beforehand about what they’re going to do and what equipment to have in place. As awareness grows, this team effort can become more mainstream and go a long way toward preventing silo explosions.

If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode